1. Field of the Invention
Polyethylene films are used extensively in agriculture as greenhouse covers, forage covers and agricultural mulch. Worldwide yearly consumption for polyethylene mulch film alone is currently over 1 billion pounds (M. H. Jensen, presentation at 27th National Agricultural Plastics Congress, 1998). Plastic mulches and row covers help retain soil moisture, increase soil temperature, inhibit weed growth, reduce insect damage and thereby increase yields (D. F. Anderson, M. A. Garisto, J. C. Bourrut, M. W. Schonbeck, R. Jaye, A. Wurzberger and R. DeGregorio, J. Sustain. Agric. 7, 39-61, 1995; J. W. Courter, University of Illinois Cooperative Extension Service Circular No. 1009, Champaign, Ill., 1969; D. E. Hill, L. Hankin and G. R. Stephens, Connecticut Agric. Exp. Sta. Bull. No. 805, New Haven, Conn., 1982). Most mulches are used for vegetable and fruit production due to their relatively high value. Disposal or recycling of polyethylene films, however, has become a daunting problem. Agricultural mulch, in particular, is very difficult to recycle due to contamination with dirt and debris as well as loss in mechanical properties from UV catalyzed oxidation. Many landfills reject mulch film because of pesticide residues and thus it must be treated as hazardous waste (B. Hofstetter, New Farm 13, 56-57, 1991). A biodegradable mulch would have the dual advantages of avoiding costs of removal and disposal as well as contributing humus to the soil.
This invention relates to a biodegradable, water resistant, agricultural mulch that is produced from paper and a cross-linked drying oil.
2. Description of the Prior Art
Several different types of degradable mulch have been considered including polyethylene film containing prooxidants (W. J. Maddever and G. M. Chapman, Proceedings of the Soc. Plast. Eng. 47th Ann. Tech. Conf., 1352-1355, 1989), starch-polyvinyl (PVOH) alcohol films (F. H. Otey, A. M. Mark, C. L. Mehltretter and C. R. Russell, Ind. Eng. Chem. Prod. Res. Develop. 13, 90-92, 1974), biodegradable polyester films (J. M. Mayer and D. L. Kaplan, Trends in Polym. Sci. 2, 227-235, 1994) and coated paper or fiber mats (J. Vandenberg and J. Tiessen, Hortscience 7, 464-465. 1972 and A. Bastiaansen, A. Hanzen, D. DeWit and H. Tournois, PCT Int. Pat. Appl. WO9609355, 1996). Although polyethylene films will disintegrate, resulting fragments may require decades to completely biodegrade, and toxicity of degradation products is largely unknown (A. C. Albertsson and S. Karlsson, J. Appl. Polym. Sci. 35, 1289-1302, 1988). Starch-PVOH films have rather poor resistance to water and thus would not be expected to maintain their integrity during rain. Progress is being made on laminating starch-PVOH films with different types of water-resistant, biodegradable polyesters (J. W. Lawton, in Cereals, Novel Uses and Processes, Plenum Press, New York, 1997, p. 43-47). Although biodegradable polyesters such as polylactic acid, polycaprolactone and polybutylene succinate have excellent mechanical properties (J. M. Mayer and D. L. Kaplan, Trends in Polym. Sci. 2, 227-235, 1994), their cost ($2-8/lb.) is much higher than for polyethylene ($0.4/lb. resin, $1-2/lb. film) (D. F. Anderson, M. A. Garisto, J. C. Bourrut, M. W. Schonbeck, R. Jaye, A. Wurzberger and R. DeGregorio, J. Sustain. Agric. 7, 39-61, 1995 and Anonymous, Plastics Technol., May, 1998, p. 87). Uncoated paper, although inexpensive ($0.28/lb. for kraft paper) (Anonymous, North American Pulp and Paper Yearbook, Resource Information Systems, Charlottesville, Va., 1996, 95), degrades too rapidly to protect most crops adequately (D. F. Anderson, M. A. Garisto, J. C. Bourrut, M. W. Schonbeck, R. Jaye, A. Wurzberger and R. DeGregorio, J. Sustain. Agric. 7, 39-61, 1995).
Various types of coatings for paper have been developed to slow degradation and improve wet strength. Rivise (C. W. Rivise, Paper Trade J. 89, 55-57, 1929), Hutchins (A. E. Hutchins, Minn. Agr. Expt. Sta. Bull. No. 298, 1933) and Flint (L. H. Flint, U.S. Dept. of Agric. Tech. Bull. No. 75, 1928), have reviewed some of the early work on the use of paper mulches. In 1870, the first U.S. patent pertaining to utilization of paper as a mulch described the use of tarred paper to exclude insects from roots (S. Brunson, U.S. Pat. No. 104,418, 1870). By the 1920's, chiefly through the work of Eckart in Hawaii on sugar cane and pineapple, the dramatic advantages of tar or asphalt coated paper for improving yields of fruits and vegetable became apparent. Paper impregnated with paraffin wax (V. Z. Tzelik, Russ. Pat. 28,223, 1930) and animal or vegetable oils ( W. A. Hall, Brit. Pat. 370,482, 1931) were also claimed for mulch use. With the advent of synthetic polymers in the 1940's and 1950's, polyethylene largely displaced paper in mulching applications, likely due to its low cost and excellent strength and flexibility.
Recently, however, there has been a resurgence in research and practical interest in coated paper mulches, probably due to concerns about disposal of polyethylene as well as the desire of organic farmers to have a natural, totally degradable mulch. Most of the coatings considered have been synthetic polymers such as polyethylene (J. W. Courter, University of Illinois Cooperative Extension Service Circular No. 1009, Champaign, Ill., 1969 and J. Vandenberg and J. Tiessen, Hortscience 7, 464-465, 1972), or various polymer latexes (G. E. Shanley and M. J. Lubar, Brit UK Pat. Appl. GB2158058, 1985; R. E. Weber and M. L. Delucia, Eur. Pat. Appl. EP454104, 1991; C. Desmarais, Can. Pat. Appl. CA2092963, 1994; R. F. Lippoldt and W. W. Woods, U.S. Pat. No. 3,427,194, 1969 and J. S. Vandemark and R. T. Seith, U.S. Pat. No. 3,939,606, 1976). Non-woven mats of cellulosic fibers and polyesters have also been considered (R. A. Clendinning, J. E. Potts and W. D. Niegisch, U.S. Pat. No. 3,850,863, 1976 and S. H. Monroe, J. A. Goettmann and G. A. Funk, U.S. Pat. No. 5,532,298, 1996). Anderson et al (D. F. Anderson, M. A. Garisto, J. C. Bourrut, M. W. Schonbeck, R. Jaye, A. Wurzberger and R. DeGregorio, J. Sustain. Agric. 7, 39-61, 1995) recently showed that the rate of loss of tensile strength of paper in soil can be slowed slightly by soaking it in soybean oil. Zhang et al (L. Zhang, H. Liu, L. Zheng, J. Zhang, Y. Du and H. Feng, Ind. Eng. Chem. Res. 35, 4682-4685, 1996) found that coating a regenerated cellulose film with a thin layer of tung oil followed by polymerization slowed weight loss in soil (half life increased from 30 to 37 days).